Electrical and stability properties and ultrasonic microscope characterisation of low temperature co-fired ceramics resistors

This paper presents systematic investigations of electrical and stability properties of various low temperature co-fired ceramics (LTCC) resistors. One of the goals of this work was to check the compatibility of LTCC materials (tapes, resistive and conductive inks) from various manufacturers. Three commercially available green tapes and three LTCC resistor/conductor systems were examined. The resistive inks with 1 kΩ/sq. nominal sheet resistance were used. Buried (inside) and surface resistors were laminated and fired according to the tape manufacturers’ recommendations. The influence of dimensional effect on sheet resistance and hot temperature coefficient of resistance, the temperature dependence of resistance in a wide temperature range (from −180°C to +130°C), long-term stability of thermally aged as-fired resistors (150°C, 500 h) and durability to high-voltage micro- or nanosecond pulses (50 ns pulses with 4000 V/mm maximum electric field or 10 μs ones with 700–1000 V/mm electrical field) were carried out for electrical and stability characterisation of LTCC resistors. Non-destructive scanning acoustic microscope diagnostics was applied for structure investigation and estimation of lamination and cofiring process quality of buried LTCC resistors.     

Stability of miniaturized non-trimmed thick- and thin-film resistors

This paper is focused on reliability tests of non-trimmed miniaturized thin-film and thick-film resistors. Thick-film resistors are screen printed by polymer paste on LTCC (Low Temperature Co-fired Ceramic) substrate by two different approaches. Nonstandard precise screen printing process provide tolerance of resistivity less than 5% and thus further trimming is not necessary. OhmegaPly material with Nickel Phosphorous (NiP) metal alloy is used for thin-film resistors fabricated by subtractive process. Miniaturized resistors have dimensions 0.5 × 0.5 mm, and thus 1 square, with thickness 1 μm for thin-film and 20 μm for thick-film resistors. Stability of miniaturized resistors were tested by humidity test, thermal shocks, long-term thermal ageing, direct current stress, current pulses and simulation of soldering process using VPS (Vapour Phase Soldering). Resistivity of resistors is measured by four wire method before and after each set of test and relative change of resistivity is plotted in graphs. Influence of every test on each type of resistor is analysed.     

Thermal fatigue endurance of collapsible 95.5Sn4Ag0.5Cu spheres in LTCC/PWB assemblies

The behavior of thermomechanically loaded collapsible 95.5Sn4Ag0.5Cu spheres in LTCC/PWB assemblies with high (LTCC/FR-4; ΔCTE 10 ppm/°C) and low (LTCC/Arlon; ΔCTE < 10 ppm/°C) global thermal mismatches was studied by exposing the assemblies into two thermal cycling tests. The characteristic lifetimes of the LTCC/FR-4 assemblies, tested over the temperature ranges of 0–100 °C and −40 to 125 °C, were 1475 and 524 cycles, respectively, whereas the corresponding values of the LTCC/Arlon assemblies were 5424 and 1575 cycles. According to the typical requirements for the industrial lifetime duration of solder joints, the former values are inadequate, whereas the latter are at an acceptable level in a few cases. Furthermore, the global thermal mismatch affected the thermal fatigue behavior of the 95.5Sn4Ag0.5Cu spheres in the temperature range of −40 to 125 °C.     

Multiple high-voltage pulse stressing of conventional thick-film resistors

In this paper, results from a study of multiple high-voltage pulse (MHVP) stressing effects on resistance and low-frequency noise of thick-film resistors based on resistor compositions with sheet resistances of 1, 10 and 100 kΩ/sq are presented. For experimental purposes, a series of thick-film test resistors with identical geometries were realized and exhibited to two types of tests: multiple series of 10 pulses with increasing amplitudes from the 0.5 to 4.0 kV range and multiple series of 10 pulses with constant 3 kV and 4.5 kV amplitudes. Obtained experimental results were analyzed and correlation between resistance and low-frequency noise changes with resistor degradation due to MHVP stressing was observed. Comparing the resistance and noise index changes it is shown that low-frequency noise parameters are more sensitive to this kind of resistor stressing than resistance. During high-voltage pulse stressing, certain number of resistors failed and progressive resistor degradation that led to catastrophic failure is also presented in this paper.     

Environmentally sustainable composite resistors with low temperature coefficient of resistance

Temperature coefficient of resistance (TCR) of thick film resistors are based on fired conducting grains and glass composites. Many analog sensor and control circuits require low (<100 ppm/°C) TCR value. To prepare resistors with low TCR value, knowledge of processing conditions and conduction mechanism parameters are of particular importance because TCR is finalised during firing and cannot be trimmed in the latter stage to a target value as resistors can be. This paper reports the preparation and properties such as microstructural and electrical in particular to sheet resistance, TCR (hot and cold) of eco-friendly composite resistor paste compositions. Our resistor compositions showed the sheet resistance in the range of 1.18–1.38 KΩ/□ and the hot and cold TCR of the compositions reduced substantially from 360 to 100 ppm/°C and 175 to 60 ppm/°C with the addition of TCR modifier.     

Microvaristors in thick-film and LTCC circuits

ZnO-based varistors protect electronic circuits against overvoltage. High temperature from the range of 1150-1300 °C is required for proper sintering of such material. Varistor inks with lower firing temperature are needed for application in thick-film and LTCC technology. ZnO-based thick-film composition was prepared and varistors were fabricated on alumina and LTCC substrate. Different topologies (capacitor-like or planar), electrode metallurgies (PdAg, Au or Pt-based) and firing profiles (850 °C or 950 °C) were used. Samples microstructure was investigated. Varistor I-V characteristics, long-term stability and durability to high voltage pulses were examined. Satisfactory results were achieved, because nonlinearity coefficient α up to 23 was obtained for capacitor-like varistors with Pt terminations on LTCC substrates, long-term thermally aged (150 h at 250 °C) varistors had slightly smaller nonlinearity coefficient and characteristic voltage, V_1 mA and components subjected to series of high voltage pulse (1000 pulses with 10 mA amplitude and 5 ms duration each) exhibited almost the same electrical parameters.     

High-voltage pulse stressing of thick-film resistors and noise

In this paper the results from a study of high-voltage pulse stressing effects on resistance and low-frequency noise of thick-film resistors based on two different resistor compositions with sheet resistances of 10 and 100 kΩ/sq are presented. For the experimental purposes thick-film test resistors of different dimensions were realized and exhibited to voltage pulses with 1500 and 3000 V amplitudes. Obtained experimental results are qualitatively analyzed from the microstructure, charge transport mechanism and low-frequency noise aspects. Correlation between resistance and low-frequency noise changes with resistor degradation due to high-voltage pulse stressing is observed. It is shown that low-frequency noise is more sensitive to this kind of resistor stressing than resistance and that measured values of noise index are in agreement with resistance noise spectrum results.     

Thick-film resistor quality indicator based on noise index measurements

A quality indicator for thick-film resistors based on noise index and resistance measurements is proposed. As this correlates resistor transport and noise characteristics and has mobility dimensions, we titled it to be noise reduced mobility. The experimental results for thick-film resistors, realized using three different resistor compositions with sheet resistances of 1, 10 and 100 kΩ m/sqr show that layers with sheet resistance of 10 kΩ0.25>m/sqr have minimum value of noise reduced mobility in comparison with layers formed using resistor compositions with sheet resistances of 1 and 100 kΩ m/sqr. The potential and resistance distributions measured along test resistors show that the noise reduced mobility is in correlation with thick-film inhomogeneity.     

Fabrication and electrical properties of laser-shaped thick-film and LTCC microresistors

The dimensions of discrete passives, passive integrated components (arrays, networks) and embedded integral ones should be reduced significantly in the nearest future. Therefore the relations between technological accuracy and limitations, minimal geometrical dimensions and electrical as well as stability properties become more and more important. This paper presents systematic studies of thick-film or LTCC microresistors made with the aid of laser shaping. The investigations are concerned with miniaturization of two resistor dimensions, namely length (down to 30 μm) and width (also down to 30 μm). The sheet resistance, hot temperature coefficient of resistance (HTCR) as well as long-term stability and durability of test structures to various short electrical pulses were related to geometrical properties of microresistors. Such investigations proved that combining of current materials and fabrication methods used in modern thick-film and LTCC technologies with laser shaping made possible fabrication of 30 × 30 μm² microresistors with satisfactory electrical properties and can serve as interesting alternative for thick-film and LTCC resistors miniaturization.     

Comparison of excess 1/f noise spectra in trimmed and untrimmed thick film resistors

Low frequency excess 1/f noise in the frequency range 1-10⁴ Hz and conductivity are measured and analysed in trimmed and untrimmed bismuth ruthenate thick film resistors (TFRs) with an aspect ratio of 0.04. The excess flicker noise exhibits a 1/f ^γ spectrum with γ ≈ 1 which scales with the square of the dc current. Although both the temperature coefficient of resistance and the conductivity of the TFRs did not change significantly upon trimming, we observed a substantial increase in the relative noise index C for trimmed TFRs. The Hooge parameter α _H for untrimmed TFRs was estimated to be ～ 1.1 × 10^?3. Application of the Hooge model to trimmed TFRs indicated that trimming increases the values of α _H. Further, if V is the volume of the TFR, the noise of the trimmed resistors did not exhibit the expected C ∝ 1/V behaviour that is observed for untrimmed TFRs. These observations may be due to a combination of reasons such as laser trimming induced inhomogeneous damage or a possible reduction in the Vandamme effective volume; trimmed TFRs exhibited an effective volume in the range 0.18-0.38.     

Fabrication and advanced electrical and stability characterization of laser-shaped thick-film and LTCC microresistors for high temperature applications

Thick-film and LTCC (Low Temperature Co-fired Ceramics) technologies are well-established and relatively low-cost fabrication method of passives. This paper presents systematic studies of fabrication and a wide spectrum of geometrical and electrical properties of thick-film and LTCC microresistors with dimensions down to 30 × 200 μm². The geometrical parameters (average length, width and thickness, relations between designed and real dimensions, distribution of planar dimensions) are correlated with basic electrical properties of resistors (sheet resistance, temperature dependence of resistance), long-term stability as well as durability of microresistors to short electrical pulses in the temperature range from 25 °C to 400 °C.     

Reliability and RF performance of BGA solder joints with plastic-core solder balls in LTCC/PWB assemblies

In this paper board-level reliability of low-temperature co-fired ceramic (LTCC) modules with thermo-mechanically enhanced ball-grid-array (BGA) solder joint structure mounted on a printed wiring board (PWB) was experimentally investigated by thermal cycling tests in the 0–100 °C and −40 to 125 °C temperature ranges. The enhanced joint structure comprised solder mask defined (SMD) AgPt pad metallization, eutectic solder and plastic-core solder balls (PCSB). Similar daisy-chained LTCC modules with non-collapsible 90Pb10Sn solder spheres were used for a reference test set. The reliability of the joint structures was analyzed by resistance measurements, X-ray microscopy, scanning acoustic microscopy (SAM) and SEM/EDS investigation. In addition, a full-wave electromagnetic analysis was performed to study effects of the plastic-core material on the RF performance of the LTCC/BGA package transition up to millimeter-wave frequencies. Thermal cycling results of the modules with PCSBs demonstrated excellent fatigue performance over that of the reference. In the harsher cycling test, Weibull’s shape factor β values of 7.9 and 4.8, and characteristic lifetime θ values of 1378 and 783 were attained for the modules with PCSBs and 90Pb10Sn solder spheres, respectively. The primary failure mode in all test assemblies was fatigue cracking in eutectic solder on the ceramic side.     

High-temperature storage and thermal cycling studies of thick filmand wirewound resistors

The operating ambient temperature for underhood automotive and aerospace applications is increasing. This work was undertaken to evaluate the suitability of thick film and wirewound resistors for distributed aircraft control systems in a 200°C-225°C operating environment. High temperature stability testing of power wirewound and thick film resistors is reported. Dale power wirewound 1 Ω, 100 Ω, and 10 kΩ resistors with power ratings of 5 W and 25 W were tested. The TCR of the 100 Ω, and 10 kΩ resistors was very small, however, the 1 Ω resistor varied by 5% over the temperature range from 25°C to 300°C. Stability with long term storage (10000 h) at 300°C was measured for the wirewound resistors unpowered and powered at 20% of rated power. With the exception of the 10 kΩ/25W resistor, the change in resistance was less than 4%. Wirewound resistors were also thermal cycled 1000 times over a temperature range from -55°C to 225°C with only one failure due to a broken internal connection. Three 900 Series thick film resistor pastes from Heraeus-Cermalloy were studied: 100 Ω/sq., 1 kΩ/sq., 10 kΩ/sq. The temperature coefficient of resistance (TCR) was measured from 27°C to 500°C in 50°C increments. The change in resistance was <±6% up to 300°C. A 2 × 2 matrix of variables was included in the 300°C storage test: untrimmed resistors, resistors trimmed up 50% in value, unpowered, and powered at 1/8 W. Palladium/Silver was the initial termination choice for these 300°C studies, but silver migration under electrical bias lead to electrical shorts between conductor traces on the substrates with powered resistors. Gold terminated thick film resistors were used for powered storage testing at 300°C. The change in resistance after 10000 h at 300°C was < 3% for all test combinations     

5 V temperature regulated voltage reference

A 5 V internally temperature regulated voltage reference integrated circuit, which achieves 0.3 ppm//spl deg/C TC over the temperature range -55/spl deg/C to 125/spl deg/C, is described. It is built using a buried zener reference in a dielectrically isolated complementary bipolar process which employs laser trimmed NiCr thin film resistors and a high thermal resistance epoxy die attach.     

Simultaneous mechanical and electrical straining of conventional thick-film resistors

In this paper results from the study of simultaneous mechanical and electrical straining effects on performances of conventional thick-film resistors based on three different resistor compositions with sheet resistances of 1 kΩ/sq, 10 kΩ/sq and 100 kΩ/sq are presented. For experimental purposes thick-film test resistors of different dimensions were realized. Resistors were simultaneously subjected to mechanical straining with maximal substrate deflection of 300 μm and multiple high-voltage pulses. Obtained experimental results are analyzed from micro- and macrostructural, charge transport and low-frequency noise aspects. Correlations between resistance, gauge factor and low-frequency noise changes with resistor degradation due to simultaneous mechanical and electrical straining are observed.     

Reliability of polysilicon thin film resistors with irreversible resistance transition

In this paper, the results of reliability testing (life testing at 125°C for 2000 h) and failure analysis of polysilicon thin film resistors with irreversible resistance transition are presented and discussed. The results of the life test show that electrical parameters of the polysilicon resistors (resistance before transition, transition voltage and resistance after transition) are satisfactorily stable during the life test. Also, calculated values of the mean time before failure (MTBF) and the mean time to failure (MTTF) on the basis of the life test data confirm that the polysilicon resistors have a satisfactory level of reliability for long-term applications. Finally, the results of failure analysis show that typical failure modes of the polysilicon resistors are open, while the responsible failure mechanism is electromigration of aluminum during the life test at the contact between the aluminum line and polysilicon film.     

Stability of low ohmic thick-film resistors under pulsed operation

Durability of low ohmic thick-film resistors on pulse load with micro- and millisecond duration are described in this paper. Standard thick-film resistive compositions with sheet resistance of 10 Ohms/sq. (4311 and QS871 - DuPont; R400-10A - Heraeus) and 3 Ohms/sq. (QS870 - DuPont) were tested. Test resistors with resistance of 3 Ω were prepared on alumina substrates. Resistance changes were measured after 50 pulses at each voltage. Then the voltage was increased and the series of pulses were repeated until the resistance change exceeded 0.5%.Impact of long and short pulses was analyzed for selected pulse duration of 10 μs and 20 ms, respectively. The measurements were carried out for some selected resistors length. The resistors stressed by long pulses exhibited the highest durability, when the thermal interface between substrate and heatsink was filled by thermally conductive grease. In this case resistors made of 4311 and QS871 pastes can withstand the highest power density of about 19 W/mm² and 18 W/mm² for the samples with and without the overglaze, respectively. Short (microsecond) pulses had the least influence on overglazed resistors made of QS871 paste, for which the resistance change of +0.5% was observed at electric field intensity of about 85 ± 10 V/mm.Additionally, the critical electric field intensity and critical power density were determined for different pulse duration for selected resistor length of 2 mm. Pulse duration from 10 μs to 1 s with one point per decade were taken into account. The results shown, that overglazed resistors exhibit better durability for all tested resistive pastes for short pulses. However for the long pulses, the results are almost the same. This fact can be explained by limitation of heat transfer in the substrate.Repetitive pulse load was applied for selected samples. The results show that resistors exhibited very good stability after test with 0.1 million pulses with amplitude of 70% of critical electric field intensity values.     

Properties of laser cut LTCC heaters

This article describes fabrication and properties of buried microheaters made inside low temperature co-fired ceramics (LTCC) structures. Laser cutting is used for meander pattern generation in dried Pt, PtAu or PdAg conductive pads. The electrical characterisation of microheaters is based on measurement and analysis of R(T) dependence in the range from 20°C to 850°C, measurement and analysis of thermal dynamic properties, long-term high-temperature passive or active ageing and behaviour of the heater in a pulse operation mode. The presented results are very promising for application of LTCC microheaters in various microsystem devices.     

Properties of low temperature Sn–Ag–Bi–In solder systems

The metallurgical and mechanical properties of Sn–3.5 wt%Ag–0.5 wt%Bi–xwt%In (x = 0–16) alloys and of their joints during 85 °C/85% relative humidity (RH) exposure and heat cycle test (−40–125 °C) were evaluated by microstructure observation, high temperature X-ray diffraction analysis, shear and peeling tests. The exposure of Sn–Ag–Bi–In joints to 85 °C/85%RH for up to 1000 h promotes In–O formation along the free surfaces of the solder fillets. The 85°C/85%RH exposure, however, does not influence the joint strength for 1000 h. Comparing with Sn–Zn–Bi solders, Sn–Ag–Bi–In solders are much stable against moisture, i.e. even at 85 °C/85%RH. Sn–Ag–Bi–In alloys with middle In content show severe deformation under a heat cycles between −40 °C and 125 °C after 2500 cycles, due to the phase transformation from β-Sn to β-Sn + γ-InSn₄ or γ-InSn₄ at 125 °C. Even though such deformation, high joint strength can be maintained for 1000 heat cycles.     

Evaluation of conductive-to-resistive layers interaction in thick-film resistors

Low-frequency noise spectroscopy is used to examine the interactions between resistive and conductive films that take place during thick-film resistor (TFR) fabrication. Two noise parameters are introduced to quantitatively describe the strength of these interactions. They refer to intensity and repeatability of the noise generated in the resistor interfaces. Extensive experimental studies performed on ruthenium dioxide and bismuth ruthenate TFRs terminated with gold, platinum–gold, palladium–silver and platinum–silver contacts from various manufacturers allow to establish criteria of pastes compatibility and to evaluate compatible systems of pastes for standard “on-alumina” and low-temperature co-fired ceramic (LTCC) resistors. It is found that gold contacts form low-size-effect, stable, low-noise interfaces both with ruthenium dioxide and bismuth ruthenate TFRs. Silver-containing terminations can be used with bismuth ruthenate but not with ruthenium dioxide resistors. Manufacturer optimized system of pastes for LTCC technology works best when used to produce high-resistive, co-fired devices.